Numerical modeling of odorant uptake in the rat and bullfrog nasal cavities

The airflow pattern in the rat and bullfrog nasal passages is determined by their complex anatomical structure. It has been found that odorant mucus solubility and flow rate strongly affect the site and amount of absorption in the rat olfactory region and bullfrog nasal cavity. Studies of olfactory gene expression have also shown that there are at least four distinct gene expression zones on the rat olfactory epithelial surface which correlate with regions of odorant uptake influenced by odorant solubility. Two anatomically accurate finite element models of the nasal cavities of the Sprague-Dawley rat and the bullfrog were constructed based on computer-generated images of nasal cast cross-sections (rat) and CAT scans of real nasal cavity (bullfrog). Both inspiration and expiration (rat only) with physiological flow rates characteristic of restful breathing and sniffing were investigated. The computed velocity field was then used to solve the convective-diffusion equation for the concentration field and odorant surface fluxes in the nasal cavity. During the rat sniffing strategy, S-shaped streamlines passing through the olfactory region were found to be less prevalent during expiratory than inspiratory flow---leading to a trapping and increase of odorant molecule concentration in the olfactory region. For the rat nasal cavity, the resting and sniffing inspiratory and expiratory mucosal uptake for three different odorants with varying mucus solubility was computed. The mass surface flux distributions were compared with the anatomic expression zones found on the rat olfactory epithelial surface. Carvone was found to be strongly absorbed onto the mucosal surface proximally for inspiratory flow (or distally for expiratory flow) while much less soluble octane was found to be more evenly absorbed throughout the cavity for flow in both directions. In general, all odorants exhibit less absorption in the lateral turbinate region on expiration since the S-shaped streamlines which ventilate this region on inspiration are not present on expiration. The relative lack of lateral olfactory flow on expiration suggests that odorant molecules will not be washed from this region on expiration. For the bullfrog nasal cavity, total amount of odorant absorbed by the mucus at a distal locus (downstream locus during inspiration) was computed for six odorants with varying mucus solubility at normal breathing and sniffing flow rates. For flow rates higher than the base flow rate (=40cm 3/min), marked increase in neural response was seen for highly soluble odorants; whereas decrease was seen for insoluble odorants. The result was seen to be reversed for flow rates lower than the base flow rate. This is in general agreement with Mozell's measurement of neural discharge in which the effect of sniff flow rate upon the magnitude of the olfactory response was investigated. Overall, the finite element numerical approach provides odorant flux information across the olfactory mucosa in the rat and bullfrog nasal cavities, and suitably relates anatomic structure and physiological olfactory function to transport data.